CD95 (CD95/Fas) is a death receptor that induces apoptosis through recruitment of the apoptosis signaling molecules FADD, caspase-8, caspase-10 and c-FLIP forming the death inducing signaling complex (DISC). We have previously demonstrated that cells can die in different ways through CD95, either dependent on (Type II) or independent of (Type I) mitochondria and recently found that only in Type I cells whereas in Type II cells an intracellular DISC forms. Recently, we demonstrated that soluble CD95 ligand (sCD95L) is only cytotoxic to Type II cells whereas in Type I cells it activates other nonapoptotic pathways. A detailed analysis of the 60 tumor cell lines of the drug screening panel of the NCI revealed that Type II cells have an epithelial genetic profile whereas Type I cells correspond to more mesenchymal tumors. The differences seen in Type I and Type II tumor cells may therefore reflect different stages of carcinogenesis, which resembles the epithelial-mesenchymal transition (EMT). Furthermore, we found that Type I and Type II cells greatly differ in their responses to two major classes of antitumor drugs (that attack microfilaments or microtubules, respectively) and that this differential sensitivity requires a functional CD95 pathway. We hypothesize that during EMT-like processes tumor cells genetically change to respond to CD95L or antitumor drugs in different ways and that the differential responses of tumor cells can be manipulated by selectively inducing or reverting EMT in Type II or Type I cells, respectively, Furthermore, we hypothesize that the reason for this differential sensitivity of tumor cells is linked to specific properties of the CD95 DISC in Type I and Type II cells. To address these hypotheses we propose the following three Specific Aims. Specific Aim #1" Determine whether induction or reversion of EMT in tumor cells causes the Type 1/11 typical changes in the CD95 pathway. Specific Aim #2" Characterize the different signal initiation complexes in Type I and Type II cells. Specific Aim #3: Identify the molecular components in Type I/Type II cells that regulate the difference in CD95 signaling. The results of this study will not only provide a better understanding of the differential signaling of CD95 in tumor cells but may also allow to design new strategies for tumor therapy by changing the CD95 cell type of tumor cells.